No person shall place or maintain upon the ground in any public place any stone, concrete, brick or other impervious material or substance in such a manner as may obstruct the free access of air and water to the roots of any tree upon any public place in the City without first having obtained a permit therefor. Unless otherwise provided for, there shall be maintained about the base of the trunk of each such tree at least nine square feet of open ground for every three inches in diameter, and for every two inches of increase of such diameter, there shall be an increase of at least one square foot of open ground.